1. Field of the Invention
The present invention relates to a line thermal printer for printing with use of a thermal head, and more particularly to a line thermal printer using a thermal head having an edge portion at which a line of heating elements is formed.
2. Description of the Related Art
There conventionally exists a line thermal printer for forming a desired print on a sheet of paper by selectively driving a plurality of heating elements arranged in a horizontal scanning direction and feeding the sheet of paper in a vertical scanning direction. An example of such a conventional line thermal printer will now be described with reference to FIGS. 7 and 8.
FIGS. 7 and 8 show an exemplary line thermal printer to be primarily used as a label printer. A plurality of labels 1 are attached to an elongated base sheet 2 to form a label sheet 3 stored in a rolled condition. The roll of the label sheet 3 is supported to a sheet support shaft 4. A sheet path 5 is provided to guide the label sheet 3 drawn from the sheet support shaft 4 along a given path. A printing section 6 is provided in connection with the sheet path 5.
The printing section 6 is composed of a platen 7 adapted to be rotationally driven by a driving member (not shown), a line type of thermal head 9 having a plurality of heating elements 8 arranged in a line, and a ribbon supply unit 14 for guiding an ink ribbon 13 along a given ribbon path 12 leading from a ribbon supply shaft 10 to a ribbon take-up shaft 11. The thermal head 9 is opposed to the platen 7 with the sheet path 5 interposed therebetween, and is supported pivotably about a fulcrum 15 to thereby come into contact with or separation from the platen 7. Further, the thermal head 9 is normally biased to the platen 7 by a biasing member (not shown). The ribbon pa th 12 passes a print position P where the heating elements 8 of the thermal head 9 come to contact with the platen 7, and the ribbon path 12 is bent at an edge portion E of the thermal head 9.
A label separating plate 16 for sequentially separating the labels 1 from the base sheet 2 by sharply bending the base sheet 2 is provided in the sheet path 5 at a position downstream of the print position P. The base sheet 2 bent by the label separating plate 16 is wound by a base sheet take-up shaft (not shown), while the labels 1 separated from the base sheet 2 are sequentially delivered from a label delivery opening (not shown).
In operation, the label sheet 3 guided in the sheet path 5 is fed by the rotation of the platen 7. During the course of such feed of the label sheet 3, desired contents such as characters and bar codes are printed on the labels 1 by the thermal head 9. More specifically, the heating elements 8 arranged in a horizontal scanning direction are selectively driven, and the label sheet 3 is fed in a vertical scanning direction, thereby transferring the ink of the ink ribbon 13 onto the labels 1 to effect printing. In printing, the ink ribbon 13 is wound by the ribbon take-up shaft 11 in synchronism with the feed of the label sheet 3, and the label sheet 3 and the ink ribbon 13 pass the print position P at the same speed.
After the label sheet 3 is allowed to pass the print position P by the rotation of the platen 7, the base sheet 2 only is wound by the base sheet take-up shaft (not shown). At this time, the base sheet 2 is sharply bent by the label separating plate 16, so that the labels 1 after printed are sequentially separated from the base sheet 2 and the labels 1 thus separated are sequentially delivered from the label delivery opening (not shown).
The related art as mentioned above has the following problems.
i) First Problem
In an exemplary structure of the related art label printer, lost feed of the label sheet 3 by a given amount is carried out to make the leading label 1 after printed reach the label delivery opening (the label separating plate 16). The lost feed is stopped when the rear end of the leading label 1 after printed just comes over the label separating plate 16, and the next label 1 is printed when the taking of the leading label 1 out of the label delivery opening is detected by a sensor or the like. In such a structure, to ensure a large effective print area on each label 1, a gap G between the adjacent labels 1 must be set wide. For example, to enable the printing from the front end position of the next label 1, the gap G between the leading label 1 and the next label 1 must be set wider at least than the amount of the lost feed of the label sheet 3. However, if the gap G is set unduly wide, the number of the labels 1 retainable in the label sheet 3 is undesirably reduced. To reduce the amount of the lost feed of the label sheet 3 for feeding each label 1 after printed from the print position P to the label delivery opening, it is considered to set the print position P close to the label delivery opening. Accordingly, even if the gap G between the adjacent labels 1 is narrow, the large effective print area on each label 1 may be ensured. However, in the conventional thermal head 9, the heating elements 8 formed at the print position P are secluded several millimeters from the edge portion E, so that it is difficult to set the print position P close to the label delivery opening.
As another technique, it is considered that after the leading label 1 is taken out of the label delivery opening, the label sheet 3 is once backward fed to carry out the printing on the next label 1. This technique is current applied. According to this technique, even if the gap G is very small or absent, the printing on the next label 1 may be started from the front end position of the next label 1. In such a structure, however, a mechanism for backward feeding the label sheet 3 must be incorporated in the printer, causing an increase in component cost and manufacturing cost of the printer to result in expensiveness of the printer. Furthermore, every time the printing on the leading label 1 is ended, the label sheet 3 must be fed backward. As a result, a period of time from the start of printing on the leading label 1 to the start of printing on the next label 1 becomes long.
While the first problem has been described in the label printer as an example, such a problem similarly occurs also in a receipt printer or the like. That is, also in the case of cutting a printed receipt with a cutter or the like and then delivering the receipt thus cut, it is necessary to perform the lost feed from the heating elements 8 of the thermal head 9 to the cutter or the like by an amount greater than the distance between the print position P and the edge portion E. As a result, the receipt paper becomes waste in its length corresponding to the amount of the lost feed.
ii) Second Problem
In the line thermal printer, it is necessary to occasionally clean the thermal head 9, so as to maintain a print quality. In cleaning the thermal head 9, the ink ribbon 13 is first removed and the thermal head 9 is then pivoted about the fulcrum 15 to be set in a head-up state. In this head-up state of the thermal head 9, the heating elements 8 separated from the platen 7 are rubbed with a brush, cotton swab, etc. to remove the stain from the heating elements 8. However, the heating elements 8 of the conventional thermal head 9 are formed at a position secluded several millimeters from the edge portion E of the thermal head 9 as mentioned above. Accordingly, even in the head-up state of the thermal head 9, the heating elements 8 are hard for an operator to see from the outside and are also hard to treat with operator's hands. Thus, a cleaning work is not easily performed. To cope with this problem, it is considered to set a large pivotable angle of the thermal head 9, thereby enabling the operator to easily see the heating elements 8 from the outside and easily treat the heating elements 8 with his/her hands. As a result, the cleaning work may be easily performed. However, a wide dead space must be defined so that the thermal head 9 pivoting at a large angle may not interfere with other members in the printer. Such a wide dead space hinders a reduction in size of the printer.
iii) Third Problem
As mentioned in First Problem and Second Problem, the heating elements 8 of the conventional thermal head 9 are formed on a plane at the position secluded several millimeters from the edge portion E of the thermal head 9. Accordingly, the thermal head 9 comes to plane contact with the platen 7, so that a nip width as a contact width between the platen 7 and the thermal head 9 is wide. As a result, a pressure applied to the thermal head 9 is dispersed. Accordingly, in order to obtain a desired printing pressure at the print position P where the heating elements 8 come to contact with the platen 7, a pressure greater than the desired printing pressure must be applied to the thermal head 9. As a result, a mechanical strength of each component must be set high to such a degree as to cope with the high pressure to be applied to the thermal head 9, thus causing a bottleneck against a reduction in size and weight and a reduction in cost of the printer. Furthermore, since the nip width is wide, a frictional area between the platen 7 and the thermal head 9 (actually, a frictional area between the thermal head 9 and the printing paper or the ink ribbon) becomes wide to increase a load to a motor for driving the platen 7. Accordingly, a large-sized motor having a high output must be used as the driving motor, thus similarly causing a bottleneck against a reduction in size and weight and a reduction in cost, and further causing a bottleneck against a reduction in power consumption.
iv) Fourth Problem
As shown in FIG. 8, a circuit board 17 for driving the heating elements 8 is mounted on the thermal head 9. The circuit board 17 is mounted on one surface of the thermal head 9 on which the heating elements 8 are formed. This is due to the fact that if the circuit board 17 is mounted on any surface other than the surface for forming the heating elements 8, lead electrodes (not shown) connected to the heating elements 8 must be bent at a corner portion of the thermal head 9 to be led to the circuit board 17. However, it is difficult to bend the lead electrodes which are formed by a thin-film technology. For this reason, the heating elements 8 and the lead electrodes connected thereto are formed on one smooth surface of the thermal head 9, and the circuit board 17 is mounted on the same surface. Then, the lead electrodes and the circuit board 17 on the same surface of the thermal head 9 are connected together without bending the lead electrodes.
However, the circuit board 17 requires an IC cover 18 for covering an IC (not shown) provided on the circuit board 17 and a connector 19 for supplying data to drive the heating elements 8. The IC cover 18 and legs 19a of the connector 19 fixed by soldering or the like to the circuit board 17 project from the surface of the thermal head 9 where the heating elements 8 are formed as shown in FIG. 8. Accordingly, the sheet path 5 must be formed so as not to interfere with the IC cover 18 and the legs 19a of the connector 19. Therefore, the sheet path 5 is bent at the print position P, so as to prevent the interference with the IC cover 18 and the like.
If the sheet path 5 is bent at the print position P, the thermal head 9 is slightly raised by the stiffness of the label sheet 3 guided in the sheet path 5. At this time, the thermal head 9 is slightly pivoted about the fulcrum 15 located upstream of the platen 7 with the result that a point PS of application of the printing pressure to the label sheet 3 by the contact pressure of the thermal head 9 against the platen 7 (which point PS will be hereinafter referred to as a printing pressure point PS) slips from the print position P (see FIGS. 9A and 9B). That is, the larger the stiffness of the label sheet 3, the more the printing pressure point PS slips downstream from the print position P. Accordingly, if the label sheet 3 having a large stiffness is used, a sufficient printing pressure cannot be obtained at the print position P to easily cause print defect such as print blur. FIG. 9A illustrate a positional relation between the print position P and the printing pressure point PS in the case where the label sheet 3 having a small stiffness is used, whereas FIG. 9B illustrates a positional relation between the print position P and the printing pressure point PS in the case where the label sheet 3 having a large stiffness is used. Such a phenomenon occurs remarkably in the case of using the label sheet 3 having a large stiffness; however, the phenomenon is not limitative to the label sheet 3, but it generally occurs in the case of using any sheet of printing paper having a large stiffness.
In these circumstances, the slippage of the printing pressure point PS from the print position P is generally prevented by increasing the printing pressure caused by the contact pressure of the thermal head 9 against the platen 7. However, such an increase in the printing pressure undesirably brings about early wearing of the heating elements 8 and necessitates an expensive high-output motor to increase a driving force for the platen 7. In another method conventionally applied, the print position P is mechanically slipped according to the stiffness of the label sheet 3 to be used, thereby making the print position P coincide with the printing pressure point PS. According to this method, however, the structure becomes complicated and the adjustment therefor is fine and difficult. Thus, this method is also undesirable.
v) Fifth Problem
In the case where the label sheet 3 is used as a sheet of printing paper as shown in FIGS. 7 and 8, it is desirable that a positional relation between an entrance 5En of the sheet path 5 and the sheet supply shaft 4 should be set so as to allow the label sheet 3 to pass the entrance 5En in a straight condition or in a bent condition where the label sheet 3 is bent to the labels 1 side. If the label sheet 3 passes the entrance 5En in a bent condition where the label sheet 3 is bent to the base sheet 2 side, the leading end of each label 1 is easily separated from the base sheet 2 at the entrance 5En to possibly cause paper jam. In this manner, the positional relation between the entrance 5En of the sheet path 5 and the sheet supply shaft 4 cannot be freely set.
vi) Sixth Problem
The ink ribbon 13 is generally classified into a cold separation ribbon and a hot separation ribbon. The cold separation ribbon is used in such a manner that when the ink melted by heat from the heating elements 8 of the thermal head 9 and transferred onto a sheet of printing paper is cooled to be solidified, the ink ribbon is separated from the printing paper. On the other hand, the hot separation ribbon is used in such a manner that while the ink melted by heat from the heating elements 8 and transferred onto the printing paper remains hot and melted, the ink ribbon is separated from the printing paper. The hot separation ribbon has advantages that high-speed printing can be effected and good transfer of the ink can be effected even onto a sheet of printing paper having a bad surface property. However, in the case of using the hot separation ribbon, when the ink melted and transferred onto the printing paper is cooled to be solidified, the ink adheres strongly to the ink ribbon rather than to the printing paper. Accordingly, if the hot separation ribbon is separated from the printing paper after the ink melted is cooled, the ink that should be fixed to the printing paper is undesirably fixed to the ink ribbon and is separated from the printing paper together with the ink ribbon, thus greatly reducing a print quality.
In the conventional thermal head 9, the heating elements 8 are located at a position secluded several millimeters from the edge portion E of the thermal head 9 as mentioned previously. Accordingly, the ink ribbon 13 cannot be separated from the printing paper immediately after the ink ribbon 13 is heated by the heating elements 8, because a front portion of the thermal head 9 on the downstream side of the heating elements 8 hinders the separation of the ink ribbon 13. Thus in the conventional thermal printer, the ink ribbon 13 cannot be separated from the printing paper while the ink remains hot and melted, and it is difficult to effect good printing with use of the hot separation ribbon.